Gas discharge lasers are used in photolithography to manufacture semiconductor integrated circuits. As semiconductor manufacturing has progressed to requiring smaller and smaller feature sizes (that is, the minimum feature size used to fabricate the integrated circuit), the design and performance of these lasers has improved. For example, gas discharge lasers have been redesigned to provide shorter wavelength and narrower bandwidth to support higher resolution, to provide higher powers to enable higher throughput, and to stabilize performance parameters such as dose, wavelength, and bandwidth.
Excimer lasers are one type of gas discharge laser used in photolithography that can operate in the ultraviolet (UV) spectral region at high average output power to generate nanosecond pulses at reduced spectral bandwidth.
In some cases, these lasers are designed with a dual chamber design having first and second chambers to separate the functions of providing narrow spectral bandwidth and generating high average output pulse energy. The first chamber is called a master oscillator (MO) that provides a seed laser beam and the second chamber is called a power amplifier (PA) or a power oscillator (PO). A master oscillator typically includes a gain medium in which amplification occurs and an optical feedback mechanism such as an optical resonator. The power amplifier typically includes a gain medium in which amplification occurs when pumped with the seed laser beam from the master oscillator. If the power amplifier is designed as a regenerative ring resonator then it is described as a power ring amplifier (PRA) and in this case, enough optical feedback can be provided from the ring design. The power amplifier receives the seed laser beam from the MO. The MO chamber enables fine tuning of parameters such as the center wavelength and the bandwidth at relatively low output pulse energies. The power amplifier receives the output from the master oscillator and amplifies this output to attain the necessary powers for output to use in photolithography. The dual chamber design can be referred to as a MOPA, MOPO, or MOPRA, depending on how the second chamber is used.